The present invention relates to dye-appended polymeric materials for use in broadband fiber optic device applications.
Dense wavelength division multiplexed (DWDM) optical networks increase their transmission capacity by employing multiple co-propagating, discrete, wavelength channels, each carrying independent data streams. Broadband fiber optic devices, such as variable attenuators, couplers, and switches having a controllable spectral response, are critical components of DWDM systems. Currently, DWDM systems operate in the 1550 nm spectral region because of the availability of optical amplifiers containing erbium-doped optical fibers. However, as amplifier technology develops, and capacity demands increase, DWDM systems are expected to expand their spectral extent and increase their channel density.
Optical power, as it propagates in a single-mode optical fiber, or any other waveguide or bulk material, experiences dispersion, i.e. differing wavelengths propagate at different speeds. In an optical fiber, modal extent and phase velocity are affected by both the dispersion of the coupling material and the dispersion of the waveguide causing the light to pass through at different speeds. Thus, across a given wavelength region, differences between the dispersions of the material and waveguide through which light propagates can result in nonuniform spectral performance of fiber-based devices.
Dispersion is often represented in terms of a material""s refractive index (n) as a function of optical wavelength (xcex), i.e. as n(xcex). In dispersive materials, the refractive index of the material changes with wavelength. The relevant parameter when describing modal dispersion or multimode distortion in optical fibers is the effective mode dispersion, neff(xcex), which, in simple waveguide geometries, can be calculated using the material dispersion of the fiber""s cladding and core, nclad(xcex) and ncore(xcex), respectively, and geometric parameters. The relationships between dispersion, refractive index, wavelength, and spectral performance for fiber optic devices and polymer overlays are fully disclosed in copending commonly assigned U.S. patent application Ser. No. 09/139,457 filed Aug. 24, 1998.
As represented herein, xe2x80x9cneff(xcex)xe2x80x9d refers to the effective mode dispersion for a silica glass optical fiber having a core with a slightly raised refractive index relative to the surrounding cladding. The term xe2x80x9cdispersionxe2x80x9d refers to the slope of the line formed from a plot of a material""s change in refractive index versus change in wavelength. Although all materials are dispersive to some extent, a hypothetical material exhibiting no dispersion would be represented as a horizontal line (slope=0). The greater the dispersion, the steeper the slope (negative or positive). The slope of neff(xcex) is negative, and thus, a single mode optical fiber is dispersive.
Fiber-based devices frequently exhibit spectrally non-uniform performance, which is undesirable in many broadband device applications. Examples include side-polished fiber (SPF)- and tapered fiber-based attenuators wherein a coupling oil (nD=1.456 at 27.9xc2x0 C.) placed on the optical fiber induces power loss (attenuation), as disclosed in commonly assigned U.S. Pat. No. 5,966,493. However, the attenuation is not uniform across the spectral region because the dispersion of the oil, noil(xcex), is mismatched to that of the fiber, neff(xcex), ie. the slope of noil(xcex) differs from that of neff(xcex). By contrast, if the dispersions were matched, corresponding dispersions would be approximately parallel, and the attenuation would be almost constant or substantially uniform across the wavelength band with only small variations being observed. Thus, a plot of attenuation (dB) vs. wavelength would result in a substantially horizontal line (slope=0) indicating uniform spectral response for dispersion-matched materials.
As disclosed in the aforementioned U.S. Pat. No. 5,966,493, certain organic polymers having an index of refraction close to that of the fiber can be applied to the exposed surface of a SPF optic (or a tapered fiber optic) for use in variable optical attenuators. Such polymers exhibit a change in refractive index proportional to a change in temperature. OPTI-CLAD(copyright)145, which is available from Optical Polymer Research, Inc. is an example of such a polymer. Although the refractive index of such organic polymer materials can be altered at a given wavelength to match that of the fiber, the use of these polymers is limited in broadband applications because of the dispersion mismatch between the polymer and the fiber across the wavelength band of interest.
One solution to these problems of dispersion-mismatch and nonuniform spectral response is provided in the aforementioned U.S. patent application Ser. No. 09/139,457, which discloses the use of polar polyolefin copolymers having certain infrared absorbing dyes incorporated therein. Surprisingly, use of these novel materials permits dispersion to be controlled from very large differences to almost no difference in dispersion between the dye/polymer composition and the fiber optic. The refractive index of the dye/polyolefin formulations disclosed in the application can also be altered to match or differ from that of the optical fiber. These dispersion-controlled, refractive index-controlled dye/polymer compositions are particularly useful in the fabrication of spectrally uniform fiber optic devices such as VOAs, couplers, and switches for use in broadband applications, such as in the 1500-1600 nm region, where control of spectral response is important.
Although the dye-doped polymer compositions disclosed in the above patent application provide an excellent solution to the dispersion matching problem for the aforementioned devices, rigorous efforts have continued with the goal of developing even better materials. For example, due to the limited solubility of the dye, attention to the amount of dye introduced into these polymers is particularly important. Phase separation of the dye from the polymer may occur if the weight percentage ranges disclosed in the application are not adhered to. Thus, it would be advantageous to develop dye/polymeric materials having improved thermal stability and phase stability. Such formulations should exhibit greater long term reliability and improved durability making them even more commercially valuable. In addition, if solubility concerns can be eliminated, then it may be possible to use other polymers, not previously employed, in the formulations. At the same time, it remains important to maintain a uniform spectral response across a broad wavelength range.
The present invention meets the aforementioned needs and is based on the unexpected discovery that certain dye-appended polymeric materials, in which the dye and polymer are chemically bonded together, eliminate solubility issues associated with doped systems. Thus, phase separation of the dye from the polymer is no longer a concern, and larger amounts of dye can be incorporated into the compositions. The present compositions provide improved control/correction of dispersion mismatch between the polymer and the fiber optic, and therefore, spectral flatness across a given wavelength band. In addition, because the issue of phase separation has been eliminated, polymers free of any dye can also be added to the dye-appended polymers of the present invention to obtain an even better (more uniform) spectral response over a broader wavelength range (e.g., 1500 nm to 1700 nm). Thus, the novel dye-appended polymer formulations of the present invention with and without additional copolymers mixed therein are particularly useful in the fabrication fiber optic devices exhibiting spectrally uniform performances, such as variable optical attenuators (VOA)s, couplers, shutters, and switches. Furthermore, the present dye-appended polymeric compositions can be used at longer wavelengths, up to about 1700 nm.
Accordingly, in one aspect, the present invention is a dye/polymer composition comprising:
(a) an infrared absorbing dye component having an absorption maximum from about 900 to about 1300 nm; and
(b) a copolymer component comprising at least one appended polar olefin copolymer chemically bonded to the dye component through a linking moiety attached to one chain end of the copolymer. Each appended polar olefin copolymer comprises monomeric units derived from two or more polar olefins having an ester, benzene or halogen substituent attached thereto.
Exemplary component concentrations are from about 0.2 to about 10% by weight of the infrared absorbing dye component and from about 90 to about 99.8% by weight of the copolymer component. The copolymer component optionally includes a detached polar olefin copolymer mixed with the dye-appended polymer, such that the aforementioned weight percentage of the copolymer component also includes that of the detached copolymer. In the detached copolymer, the monomeric units are also derived from two or more polar olefins having an ester, benzene or halogen substituent attached thereto.
Considering both embodiments, the dye/polymer composition of the present invention therefore contains:
from about 10 wt.% to about 100 wt. % of the dye-appended polar olefin copolymer, which comprises the dye moiety chemically bonded to one or more of the appended polar olefin copolymers through a linking moiety); and
from about 0 to about 90 wt. % of the detached polar olefin copolymer.
In another aspect, the present invention is an optical device comprising a portion of an optical fiber through which optical energy can propagate. The portion of the optical fiber has a surface through which at least some of the optical energy can be extracted, and the novel dye/polymer composition overlies this surface. The spectral response of the optical device across a wavelength band of interest may be controlled by controlling the material dispersion relative to the effective mode dispersion. For uniform spectral response, the material dispersion substantially matches the effective mode dispersion.